The present invention relates generally to methods and apparatuses for imaging objects in turbid media and relates more particularly to a novel method and apparatus for imaging objects in turbid media.
As can readily be appreciated, there are many situations in which the detection of an object in a turbid, i.e., highly scattering, medium is highly desirable. For instance, the detection of a plane in fog is highly desirable for obvious reasons. In addition, the detection of a tumor within an organ of the human body, such as the breast, is advantageous since the early detection of said tumor is useful in devising effective treatment protocols. Although X-ray techniques do provide some measure of success in detecting objects in turbid media, they are not well-suited for detecting very small objects, e.g., tumors less than 1 mm in size, or for detecting objects in thick media. In addition, X-ray radiation can present safety hazards to a person exposed thereto. Other tumor detection techniques involving the use of ultrasound, magnetic resonance and radio isotopes are similarly limited in their detection capabilities and/or create safety concerns.
An alternative technique used to detect objects in turbid media, particularly tumors in the body, is direct shadowgram imaging. Historically, in direct shadowgram imaging, visible or near infrared (NIR) light is incident on one side of a medium, and the transmitted light emergent from the opposite side of the medium is used to form a transillumination image. Alternatively, the backward propagating light emergent from the same side of the medium may be used to form a back-propagation image. A difference in optical properties, such as absorption, emission, or scattering between the object and the turbid medium, provides the basis for the formation of an image. Objects embedded in the medium typically absorb the incident light and appear in the image as shadows. Unfortunately, the usefulness of traditional direct shadowgram imaging as a detection technique is severely limited in those instances in which the medium is thick or the object is very small. This is because light scattering within the medium contributes to noise and reduces the intensity of the unscattered light used to form the shadow image.
To improve the detectability of small objects located in a turbid medium using direct shadowgram imaging, many investigators have attempted to selectively use only certain components of the transilluminating (or back-propagating) light signal. This may be done by exploiting the properties of photon migration through a scattering medium. Photons migrating through a turbid medium have traditionally been categorized into three major signal components: (1) the ballistic (coherent) photons which arrive first by traveling over the shortest, most direct path; (2) the snake (quasi-coherent) photons which scatter only slightly and arrive after the ballistic photons and which deviate, only to a very slight extent, off a straight-line propagation path; and (3) the diffusive (incoherent) photons which experience comparatively more scattering than do ballistic and snake photons and, therefore, deviate more considerably from the straight-line propagation path followed by ballistic and snake photons.
Because the ballistic and snake photons represent comparatively less distorted image information than do the diffusive photons, one approach to improving direct shadowgram imaging has been to selectively use the ballistic and snake photons to form the shadowgram image. This typically involves using various space-gating, time-gating or space/time-gating techniques to permit the detection of ballistic and snake photons, while rejecting diffusive photons. Examples of this approach are disclosed in the following patents and publications, the disclosures of which are incorporated herein by reference: U.S. Pat. No. 5,140,463, inventors Yoo et al., which issued Aug. 18, 1992; U.S. Pat. No. 5,143,372, inventors Alfano et al., which issued Aug. 25, 1992; U.S. Pat. No. 5,227,912, inventors Ho et al., which issued Jul. 13, 1993; U.S. Pat. No. 5,371,368, inventors Alfano et al., which issued Dec. 6, 1994; U.S. Pat. No. 5,644,429, inventors Alfano et al., which issued Jul. 1, 1997; U.S. Pat. No. 5,710,429, inventors Alfano et al., which issued Jan. 20, 1998; U.S. Pat. No. 5,719,399, inventors Alfano et al., which issued Feb. 17, 1998; Gayen et al., xe2x80x9cSensing lesions in tissues with light,xe2x80x9d Optics Express, 4:475-80 (1999); Gayen et al., xe2x80x9cTwo-dimensional near-infrared transillumination imaging of biomedical media with a chromium-doped forsterite laser,xe2x80x9d Appl. Opt., 37:5327-36(1998); Gayen et al., xe2x80x9cNear-infrared laser spectroscopic imaging: a step towards diagnostic optical imaging of human tissues,xe2x80x9d Lasers in the Life Sciences, 37:187-198 (1999); Gayen et al., xe2x80x9cTime-sliced transillumination imaging of normal and cancerous breast tissues,xe2x80x9d OSA Trends in Optics and Photonics Series Vol. 21 on Advances in Optical Imaging and Photon Migration (""98), pages 63-66 (1998), edited by J. G. Fujimoto and M. S. Patterson, Optical Society of America; Dolne et al., xe2x80x9cIR Fourier space gate and absorption imaging through random media,xe2x80x9d Lasers in the Life Sciences, 6:131-41 (1994); Das et al., xe2x80x9cUltrafast time-gated imaging in thick tissues: a step toward optical mammography,xe2x80x9d Opt. Lett., 18:1002-03 (1993); Hebden et al., xe2x80x9cTime-resolved imaging through a highly scattering medium,xe2x80x9d Appl. Opt., 30:788-94 (1991); Demos et al., xe2x80x9cTime-resolved degree of polarization for human breast tissue,xe2x80x9d Opt. Commun., 124:439-42 (1996).
An alternative approach to the direct shadowgram imaging techniques described above has been to make use of the diffusive photons which, although containing comparatively less of the direct signal information than the ballistic and snake photons, are more abundant than the ballistic and snake photons. An example of such a technique that makes use of the diffusive photons for imaging involves inverting the experimental scattering data obtained from various points in the medium using an inverse reconstruction algorithm. Examples of inverse reconstruction techniques are disclosed in the following patents and publications, all of which are incorporated herein by reference: U.S. Pat. No. 5,931,789, inventors Alfano et al.. which issued Aug. 3, 1999; U.S. Pat. No. 5,813,988, inventors Alfano et al., which issued Sep. 29, 1998; Cai et al., xe2x80x9cOptical tomographic image reconstruction from ultrafast time-sliced-transmission measurements,xe2x80x9d Appl. Opt., 38:4237-46 (1999); Cai et al., xe2x80x9cTime-resolved optical diffusion tomographic image reconstruction in highly scattering turbid media,xe2x80x9d Proc. Natl. Acad. Sci. USA, 93:13561-4 (1996); Arridge, xe2x80x9cThe Forward and Inverse Problems in Time Resolved Infra-Red Imaging,xe2x80x9d in Medical Optical Tomography: Functional Imaging and Monitoring, SPIE, Vol. IS11, G. Muller ed., 31-64 (1993); Singer et al., xe2x80x9cImage Reconstruction of the Interior of Bodies That Diffuse Radiation,xe2x80x9d Science, 248:990-3 (1993); Barbour et al., xe2x80x9cA Perturbation Approach for Optical Diffusion Tomography Using Continuous-Wave and Time-Resolved Data,xe2x80x9d Medical Optical Tomography: Functional Imaging and Monitoring SPIE Institutes, Vol. IS11, G. Muller ed., 87-120 (1993); and J. Schotland et al., App. Opt., 32:448 (1993).
Although the various approaches described above have enjoyed some measure of success, there is considerable room for additional improvements.
It is an object of the present invention to provide a new method and apparatus for imaging objects in turbid media.
It is another object of the present invention to provide a method and apparatus as described above that represents an improvement in image quality with respect to existing direct shadowgram and inverse reconstruction imaging techniques.
It is still another object of the present invention to provide a method and apparatus as described above that has applicability in the detection of tumors and other abnormalities in human body parts, such as the breast, brain, bladder, blood, bones, cervix, aerodigestive tract, artery, liver, prostate and skin.
It is still yet another object of the present invention to provide a method and apparatus that does not involve the use of X-rays or other ionizing. radiation.
The present invention is based, in part, on the principle that the light absorption and light scattering properties of normal and diseased tissues are distinguishable when illuminated at appropriate illuminating wavelengths and that such differences can be exploited to detect tumors (or other lesions) inside the tissue using optical imaging techniques, such as direct shadowgram imaging and inverse reconstruction imaging.
Accordingly, a prerequisite to performing the present technique is to determine the appropriate wavelength(s) at which such differences are apparent between the normal and diseased tissues. This may be done, for example, by observing the optical properties of the various tissues while illuminating the tissues at a variety of different wavelengths, for example, by illuminating the tissues with an illuminating source whose output wavelength is tunable over a broad wavelength range. Examples of optical properties which may be wavelength-dependent include absorption mean. free path la, scattering mean free path ls, and transport mean free path ll. Two different types of tissues, two different types of tissue components, or a normal tissue and a tumor will have the same values for the aforementioned optical properties over most wavelengths but will have substantially different values for one or more particular wavelengths. Benign tumors and malignant tumors may also be distinguishable at certain wavelengths.
In one embodiment, the method comprises illuminating at least a portion of the turbid medium with substantially monochromatic light of at least two wavelengths in the 600-1500 nm spectral range. A first of the at least two wavelengths is equal to a resonance wavelength for an optical property of an object in the illuminated portion of the turbid medium but is not equal to a resonance wavelength for the turbid medium. A second of the at least two wavelengths is not equal to a resonance wavelength for either the object or the turbid medium. Light emergent from the turbid medium following each of the foregoing illuminations comprises a ballistic component, a snake component and a diffuse component. A direct shadowgram image may be obtained by preferentially passing from the emergent light, following each illumination, the ballistic and snake components thereof and detecting the preferentially passed light. Direct shadowgram images with better contrast may be obtained by using the ratio or difference of the shadowgram images recorded at the resonant and nonresonant wavelengths.
Alternatively, an inverse reconstruction image may be obtained by determining, following each illumination, the intensity of the diffuse component at a plurality of points in time and then using these pluralities of intensity determinations and a mathematical inversion algorithm to form an image of the object in the turbid medium.
Yet another way of obtaining an inverse reconstruction image is to use a sequence of two-dimensional (2-D) images recorded using different temporal intervals (or xe2x80x9cslicesxe2x80x9d) of the light emergent (transmission and back-propagation) from the sample, and a mathematical inversion algorithm to form an image of the object in the turbid medium. The 2-D images are spatial intensity distributions of emergent light at different points in time, I(x,y,t), of the emergent light, and the reconstructed image is a three-dimensional (3-D) image that not only detects the object but also provides its location.
An important feature of this invention is that images recorded using light of a resonance wavelength of an optical property of an object inside a turbid medium (or of a constituent of the turbid medium) can be used to map the spatial distribution of the object within the medium (or of the constituent with that specific property within the medium). For example, images of the female human breast obtained using light in the wavelength range of 1120-1240 nm can be used to map the distribution of fatty tissue therewithin (xe2x80x9ca fat treexe2x80x9d). Similarly, imaging with light in the 650-780 nm range can be used to map the deoxyhemoglobin (Hb) distribution within the breast (xe2x80x9ca deoxyhemoglobin treexe2x80x9d), imaging with light in the 820-900 nm range can be used to map the oxyhemoglobin (HbO2) distribution within the breast (xe2x80x9can oxyhemoglobin treexe2x80x9d) and imaging with light in the 940-1010 nm and 1400-1500 nm ranges can be used to map the water distribution in the breast (xe2x80x9ca water treexe2x80x9d).
Obtaining the ratio or difference of 2-D (or 3-D) images recorded using a resonant wavelength and a non-resonant wavelength will provide 2-D (or 3-D) maps of the optical property to which said resonant wavelength is resonant with, and the images thus obtained will have a much higher contrast than those obtained using a single wavelength.
A particularly useful application of the above-alluded to spectroscopic difference imaging technique is to use the changes in absorption by oxyhemoglobin and deoxyhemoglobin for monitoring blood oxygenation, which enables monitoring of body function. As seen below in FIG. 1(b), the isobestic point where the absorption coefficients of oxyhemoglobin and deoxyhemoglobin are equal is around 800 nm. At shorter wavelengths in the 670-790 nm range, the absorption coefficient of deoxyhemoglobin is significantly higher than that of oxyhemoglobin while at longer wavelengths (810-900 nm), oxyhemoglobin has a higher absorption coefficient than does deoxyhemoglobin. The spectroscopic difference image obtained by subtracting an image recorded at 800 nm (isobestic point) from one recorded at a wavelength with significantly higher absorption by Hb (670-790 nm range) can be used to provide the Hb distribution with higher contrast than a single image recorded using a wavelength in this range. Similarly, the spectroscopic difference image obtained by subtracting an image recorded at 800 nm from one recorded at a wavelength with significantly higher absorption by HbO2 (810-900 nm range) can be used to provide the HbO2 distribution with higher contrast than an image obtained using light of a wavelength in the range.
The illumination of the turbid medium and the detection of the desired components of the emergent light can be performed in either a transmission geometry or a backscattering geometry.
Additional objects, as well as features, aspects and advantages of the present invention, will be set forth, in part, in the description which follows and, in part, will be obvious from the description or may be learned by practice of the invention. In the description, reference is made to the accompanying drawings which form a part thereof and in which is shown by way of illustration specific embodiments for practicing the invention. These embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.